Harnessing Na 2 C 2 O 4 as a Dual-Function Presodiation Reagent for High-Performance Sodium-Ion Hybrid Capacitors

Sodium-ion hybrid capacitors (SIHCs) represent a promising class of energy storage devices that bridge the gap between batteries and supercapacitors by combining high energy and power densities. Nevertheless, their practical application is hindered by the irreversible consumption of sodium ions during the formation of the solid electrolyte interphase (SEI) on the anode, which severely limits the sodium inventory and, consequently, degrades capacity retention and energy density. In this work, we prepared a high-efficiency presodiation material by uniformly compositing Na2C2O4 with Super P through an ingenious spray-drying technique (S–SP/NCO). This composite not only compensates for the irreversible sodium loss via its electrochemical decomposition during the first charge but also actively promotes the formation of a robust, inorganic-rich SEI layer. When incorporated into SIHCs assembled with a hard carbon anode and an activated carbon cathode, the additive markedly improves electrochemical performance, manifested by higher Coulombic efficiency, increased specific capacity, and superior cycling stability. Moreover, the capacitor with S–SP/NCO can achieve an energy density of 149.9 Wh kg–1 while still retaining 54.77 Wh kg–1 at a high power density of 3725.85 W kg–1, outperforming the additive-free counterpart. Ex situ characterization reveals that the introduction of sodium ions optimizes the SEI composition, contributing to an inorganic-rich electrode–electrolyte interface and facilitated ion transport. This study demonstrates a facile synthetic route for fabricating a composite material with a low decomposition potential and high utilization efficiency, offering new perspectives for the design of high-performance presodiation reagents.